Here is the full list of all 42 Decay: The Mare achievements. It takes around 3-4 hours to unlock all of the achievements on Windows.
Complete the second episode.
Complete the puzzle with the plates in episode 2.
Find Jack's body.
Put the four nails in Teddy.
Find all the nails.
Collect all coins in the second episode.
Talk to Jack.
Unlock the metal box in episode 2.
Complete the second code lock puzzle in episode 2.
Find all the hidden text in episode 2.
Complete the buttons puzzle.
Unlock the first door in episode 2.
Take your first shot with the camera.
Complete the first episode.
Collect all coins in the first episode.
Complete the clock puzzle.
Complete the drawings puzzle.
Visit the hidden room with the empty chair a second time.
Find your way through the maze.
Finish the conversation with the bag in episode 1.
Break the brick wall.
Get the sledgehammer.
Complete the fan puzzle.
Meet the red butterfly.
Complete the blinking eyes puzzle.
Pick up an item.
Get out of your own room.
Get the table near all the photos to break.
Unlock the box with the circles in episode 3.
Collect all of the balls in episode 3.
Complete the puzzle with the coloured balls.
Watch the scariest movie ever!
Complete the puzzle with the symbols in episode 3.
Get the knife in episode 3.
Find Tory in episode 3.
Collect all coins in the third episode.
Complete the third episode.
Get the good ending in episode 3.
Get the bad ending in episode 3.
Use the pocket watch to travel in time.
Reveal the history of Rafael.
Use the magnifier on the painting in episode 3.